1. Field of the Invention
This invention pertains to an oil recovery method, and more specifically to a method for recovering oil or petroleum from a subterranean viscous petroleum containing formation such as a tar sand deposit in which a fluid comprising superheated steam and air is introduced into the formation to displace the oil.
2. Description of the Prior Art
There are known to exist throughout the world many subterranean petroleum containing formations from which the petroleum cannot be recovered by conventional means because of the relatively high viscosity thereof. The best known of such viscous petroleum containing formations are the so-called tar sands or bituminous sand deposits. The largest and most famous such deposit is in the Athabasca area in the northeastern part of the Province of Alberta, Canada which is known to contain over 700 billion barrels of petroleum. Other extensive deposits are known to exist in western part of the United States, and Venezuela, and lesser deposits in Europe and Asia.
Tar sands are frequently defined as sand saturated with a highly viscous crude petroleum material not recoverable in its natural state through a well by ordinary production methods. The hydrocarbon contained in tar sand deposits are generally highly bituminous in character. The tar sand deposits are generally arranged as follows. Fine quartz sand is coated with a layer of water and the bituminous material occupies most of the void space around the wetted sand grains. The balance of the void volume may filled with connate water, and occasionally a small volume of gas which is usually air or methane. The sand grains are packed to a void volume of about 35%, which corresponds to about 83% by weight sand. The balance of the material is bitumen and water. The sum of bitumen and water will almost always equal about 17% by weight, with the bitumen portion varying from about 2% to around 16%.
It is an unusual characteristic of tar sand deposits that the sand grains are not in any sense consolidated, that is to say the sand is essentially suspended in the solid or nearly solid hydrocarbon material. The API gravity of the bitumen usually ranges from about 6 to about 8, and the specific gravity at 60.degree. F. is from about 1.006 to about 1.027. Approximately 50% of the bitumen is distillable without cracking, and the sulfur content averages between 4 and 5% by weight. The bitumen is also very viscous, and so even if it is recoverable by an in situ separation technique, some on-site refining of the produced petroleum must be undertaken in order to convert it to a pumpable fluid.
Bitumen may be recovered from tar sand deposits by mining or by in situ processes. Most of the recovery to date has been by means of mining, although this is limited to instances where the ratio of the overburden thickness to tar sand deposit thickness is economically suitable, generally defined as one or less. In situ processes have been proposed which may be categorized as thermal, such as fire flooding or steam injection, and steam plus emulsification drive processes. Generation of thermal heat necessary to mobolize the bitumen by means of a subterranean atomic explosion has been seriously considered, although has not yet been attempted.
Despite the many proposed methods for recovering bitumen from tar sand deposits, there has still been no successful exploitation of such deposits by in situ processing on a commercial scale up to the present time. Accordingly, there is a definite need in the art for a satisfactory in situ combustion process, and especially in view of the enormous reserves present in this form which are needed to help satisfy present energy needs, there is a substantial need for a workable method for recovery of bitumen from tar sand deposits.